1. Field of the Invention
This invention relates to compact zoom lenses, and more particularly to compact zoom lenses of large zoom ratio with a reduced weight of the entire lens system suited to still cameras and video cameras.
2. Description of the Related Art
An example of the conventional comparatively high range zoom lens adapted to be used in a photographic camera or video camera is shown in FIG. 1, and comprises a first unit 11 of positive refractive power for focusing, a second unit 12 of negative refractive power movable for zooming in a mono-direction throughout, a third unit 13 of negative refractive power movable for zooming in a forward-and-rearward direction to compensate for image shift, a fourth unit 14 of positive refractive power which is not always necessary but is stationary during zooming so that the emerging rays are almost afocal, and a fixed fifth unit 15 of positive refractive power. A diaphragm 16, in most cases, is arranged in between the third and fourth, or fourth and fifth units. This type of zoom lens has an advantage that the physical length of the entire system can be minimized because the spaces in which the second and third units move for zooming respectively can be partly overlapped. To achieve a further minimization of the physical length, the refractive power of the second unit 12 must be strengthened as much as possible so that the total zooming movement of the second unit 12 is much reduced. The increase in the power of the second unit 12, however, gives rise to problems that fluctuation of aberrations with zooming is increased, and that the manufacturing accuracy is required to be set at a higher level. Besides these, the long-time investigation and experience of the inventor has proven that when the power of the second unit is too strong, the lens thickness must be increased in view of the minimum acceptable edge thickness of the lens, and, in some cases, therefore, the physical length of the entire system is, on the contrary, increased. Hence, the refractive power of the second unit cannot be greatly strengthened.
As for an increase of the angular field by shortening the minimum focal length, because the oblique pencil comes in on determination of the diameter of the first unit, the wider the angle of field, the larger the diameter of the first unit becomes. Thus, the entire system is increased in the lateral direction. On the other hand, when the focal length for the wide angle end is shifted to longer ones, whichever, the oblique light bundle near or at the wide angle end, or the axial maximum light bundle at the telephoto end, has a larger diameter and determines the diameter of the first unit. With the prescribed zoom ratio when preserved, the focal length for the telephoto end becomes too long. This leads to an increase in the diameter of the first unit.
Another disadvantage of the type of zoom lens shown in FIG. 1 is that because the second and third units both are negative in refractive power, the axial rays emerging from the zoom section diverge so that the diameter of the aperture of the diaphragm that follows it tends to increase. In general, it is in the nature of zoom lenses that as the focal length increases, the diameter of the aperture of the diaphragm increases to maintain the constant speed of the entire system. Also, the required diameter of the casing for the diaphragm is equal to about 2 times the maximum value of the aperture diameter. When in designing a compact lens, it is, therefore, not preferable to choose such a type that the longest focal length of the entire system should be shifted toward longer ones.
Meanwhile, Japanese Patent Publication No. SHO 51-12424 has proposed another type of 5-unit zoom lens in which as shown in FIG. 2, the sign of refractive power of the third unit 23 is changed to positive, and the direction of its movement is made monotonous and opposite to that of movement of the negative second unit 22 in order to achieve an increase in the zoom ratio. In FIG. 2, 21 is the first unit for focusing; 24 is the fourth unit for making almost afocal the rays of light passed through the first to third units; 25 is the fifth unit having the image forming function; 27 is an optical member for splitting off part the light to a finder; 26 is the diaphragm. In the zoom lens of FIG. 2, the rays of light emerging from the third unit become somewhat convergent, thereby giving an advantage that the diaphragm is sufficient with a comparatively small maximum diameter of aperture. However, in order to reduce the diameter of the first unit, the separation between the second and third units must be somewhat increased. As a result, the physical length of the entire system becomes longer. Also, the diameter of the third unit is caused to increase. In order to reduce the physical length of the entire system, the refractive powers of the second and third units may be strengthened to decrease the total zooming movements of both of them. But, the increase in the power leads to increase the fluctuation of the aberration with zooming. On this account, the power cannot be too much strengthened.